This invention relates to a borehole tool, and, more particularly, to a method and apparatus for performing the downhole operation of injecting a fluid into the wall of a wellbore through a perforation made by the tool. The present invention has been developed in response to a particular problem involving a squeeze cementing operation in large diameter wellbores. Therefore, while the methods and apparatus disclosed herein would lend themselves to any operation involving injection of a fluid into the formation, the disclosure for the most part, and particularly the background of the invention, will relate to a squeeze cementing operation.
Oil and gas well cementing is a process of mixing a cement-water slurry and pumping it down through steel casing to critical points located in the annulus around the casing. Cementing a well helps provide protection against salt water flow for possible productive zones behind the casing, thus conserving the producing formation's value. Also the cement helps provide protection against corrosion of the borehole casing from subsurface mineral waters and electrolysis from the outside. In addition, cementing reduces the danger of the fresh water strata being contaminated by oil and gas or salt water flow. It also reduces the danger of a blow out caused by high pressure gas zones behind a casing and from collapsing casing caused by tremendous external pressures inherently encountered. Cementing operations for protection against the above-described downhole conditions are called primary cementing. Another type of cementing operation effected during an oil or gas well's life is secondary cementing. Secondary cementing deals with the completion and remedial repairs on a well after the producing zone is reached.
Squeeze cementing is the most common type of remedial (secondary) cementing. The process includes the utilization of hydraulic pressure to force, or squeeze, a cement slurry into contact with a formation, either in open hole or through perforations in the casing or liner. A wide selection of various types of prepared oilwell cements exists in the prior art. Adjustable water-cement ratios and various admixes provide a very flexible process for solving many problems of a corrective or remedial nature in producing oil or gas wells. pg,3
In many conditions the cement slurry may be applied to water or oil or gas bearing portions of a producing zone to eliminate excessive water or gas without sealing off the oil. This process is especially beneficial in correcting defects in producing wells. For example, where there is a problem of high gas/oil ratios, squeeze cementing can be used where an oil zone can be isolated from an adjacent gas zone, so that the gas/oil ratio can usually be improved to help increase oil production. Another example of its use is in the production of excessive water. In this case water sands can be squeezed off below the oil sand to help improve water/oil ratios. Additionally, independent water zones can usually be squeezed to eliminate water intrusion into a wellbore.
Numerous other prior art uses for squeeze cementing exist. A casing leak may be repaired by squeezing cement through the damage area. Low pressure zones that imbibe oil, gas, or drilling fluids can usually be sealed by squeeze cementing. Channeling or insufficient annular fillup behind the casing can usually be overcome by squeeze cementing. Greater protection against fluid migration into the producing zone is often possible by perforating below, squeezing perforations, repeating the process above the zone, drilling out and then perforating for production. In wells having a multiple producing zone potential, it is a common practice to isolate a zone for production and produce it to depletion. After squeezing the depleted zone, the remaining zones are, in turn, perforated, produced, depleted and plugged. In addition squeeze cementing is sometimes employed to seal off perforations or plug a depleted open hole producing zone. This helps prevent fluid migration to and from the abandoned zone.
Two prior art methods of squeeze cementing that will be described are the brandenhead method and packer method. In the bradenhead method, cement is pumped into the cased hole through tubing or drill pipe, displacing well fluids into the annulus. After the cement is placed across the zone to be squeezed, the tubing is pulled above the perforations and the annulus is closed at the surface. As pumping of cement continues, the cement moves into the zone. Circulation of the annulus is limited by the closed hydraulic system. After the cement is displaced, the slurry remaining in the casing can sometimes be reversed out. Usually however drilling is required to remove the cement. Since no packer is used, only low pressure squeezes are permitted because of casing limitations. Pinpoint accuracy of spotting the cement across the interval to be squeezed is then difficult to obtain because no packers are used.
The packer method is generally considered to be superior to the bradenhead method. The interval to be squeezed is isolated from the surface by a packer run and set on tubing. Many types of packers are conventionally available, each designed for specific well conditions, and either retrievable or permanent packers can be used. In certain instances it is necessary to isolate the section below the perforations to be squeezed. A bridge plug is placed below the perforations for this purpose. The upper perforations are then squeezed and the remaining slurry reverses out.
The packer method permits high squeeze pressures and permits more efficient placement of the slurry. However, the packer method involves the use of commercially available packers which are normally available only up to a casing size of 13 and 3/8ths inches. Problems thus arise in boreholes of larger diameters. Additionally, the packer operation is typically complex due to setting bridge plugs below perforations and the packers above. Finally, when using a squeeze packer, it is of critical importance to pressure-test the squeeze area including the packer seal and tubing and casing leaks. Even small leaks in the system can cause rapid local dehydration of the slurry and a false indication of the squeeze progression. It may thus be seen that for larger diameter casings (on the order of 36 inches) seal integrity around the squeeze is of tantamount import and prior art methods and apparatus have proven inadequate.
It would be an advantage thereof to provide a method and apparatus for squeeze cementing boreholes of relatively large diameter which could overcome the problems of the prior art. The method and apparatus of the present invention provides such a system. A downhole device is provided for utilization in any size wellbore wherein select penetration of the wellbore casing must be effected. Delivery of the slurry is herein effected through narrow conduit in closed communication with the downhole device which sealably engages the casing about the point of penetration. In this manner casing packers may be eliminated and post squeezing redrill of plugs obviated.